rtrialler- 



'umortierite. 



elliptical and may become very irregular in places. The sphe- 

 rulites consist of fibers radially arranged and show all the 

 optical phenomena of " spherulites." The dumortierite has 

 parallel extinction and its birefringence is somewhat higher 

 than that of the quartz and also slightly more than that of the 

 andalusite, though the difference between that of the latter two 

 minerals is very small. 



The intensity of the pleochroism of the dumortierite varies 

 so that in some spherules there are concentric bands of fibers 

 varying greatly in the intensity of their color. Some are 

 almost colorless, and it was at first thought they might be par- 



allel growths of andalusite with the dumortierite, but such a 

 conclusion could not be verified. 



The fibers are not always perfectly radial. They are at 

 times gathered into " brushes " and a number of these put 

 together may form a spherulite. The fibers are thus more 

 thickly crowded in places than elsewhere, and this frequently 

 results in intensifying the pleochroism, so that in some spheru- 

 lites there are numerous blotches of blue much deeper colored 

 than the rest. Muscovite is frequently abundant in a spheru- 

 lite, being formed between the fibers, and is probably an altera- 

 tion product of the dumortierite. Frequently a mass of dumor- 

 tierite will be almost completely changed to mica, leaving but 

 small fragments of the original mineral behind. 



The fibers of dumortierite, while usually arranged radially, 

 sometimes assume different shapes, and some of the masses of 

 dumortierite seen under the microscope are reproduced in fig. 2. 



